It's a free market -- tuition keeps going up because you, your neighbors, and every other American, is willing to pay for it. Prices will keep going up until people are so strained that they're not willing or able to pay anymore.
The parents of international students, primarily from Asia, are also willing to pay for it. The US has recently jumped all out onto this income stream (since 2008), and if you look at how things went in countries that have been doing it much longer, like Australia, you'll realize it'll be quite a while before Asian parents aren't willing and able to pay US college tuition for their children anymore.